Plasmas consist of gaseous complexes in which atoms or molecules are dissociated into free electrons, ions, free radicals, and neutral particles; stars, for instance, consist predominantly of plasmas. On earth, plasma occurs naturally in lightning bolts, flames, and similar phenomena, or may be manufactured by heating a gas to high temperatures, or by applying a strong electric field to a gas. Plasmas are called the “fourth state of matter” because their physical properties make them physically distinct from solids, liquids, and gases.
Ions, as well as electrons, from various kinds of plasma generators can be used in such industrial processes as etching, ashing (as with photoresist material or surfaces being chemically machined), deposition of materials such as oxides or nitrides, oxidation, sputtering, polymerization, ion implantation within surfaces and in high-specific-impulse thrusters for use on satellites and other space vehicles.
Drawbacks of existing direct current (DC) ion sources include erosion, short service life of plasma generators, and plasma non-uniformity. Erosion derives from the impacting of high-speed ions on the surfaces of the machines that produce plasmas. For example, DC ion sources eject erosion products into the discharge plasma as a consequence of the fact that the discharge cathode is constantly being bombarded by the ions of the plasma in which it is immersed. This is an undesirable attribute from the standpoint of materials processing, as contamination of the work product can result. DC ion sources (and DC electron sources) have limited lifetimes due to being constantly subjected to erosion, and the cathodes that drive such plasma sources typically, over time, lose their ability to emit electrons so that eventually the cathodes fail. Typically, DC ion sources (ion thrusters in particular) utilize a single on axis discharge cathode. This arrangement gives rise to peaked, non-uniform plasma density profiles at the exit plane. Such non-uniform profiles give rise to non-uniform wear of the ion extraction grids—thereby leading to failure by structural degradation or by electron backstreaming.
Disk shaped multi-slotted antenna designs have been used in the past to circumvent issues described above. These sources require, however, an insulating window for operation, i.e., for impedance matching and shielding. The insulating window, typically boron nitride makes such devices impractical for ion sources or ion thruster applications because the insulating window will acquire a coating over time due to wear of the extraction grids. The coating will ultimately prevent microwaves form penetrating the source and thus plasma production will cease.
Prior art U.S. patents include U.S. Pat. No. 6,376,796 issued to Sato, et al entitled “Plasma processing system”; U.S. Pat. No. 6,190,496 issued to DeOrnellas, et al entitled “Plasma etch reactor and method for emerging films”; U.S. Pat. No. 6,033,481 issued to Yokogawa et al entitled “Plasma processing”; and U.S. Pat. No. 5,891,252 issued to Yokogawa et al entitled “Plasma processing apparatus.”
Sato, et al., U.S. Pat. No. 6,376,796 relates to a plasma processing system utilizing an antenna supplying high power generating high density plasma for performing processing on the surface of a substrate. A slotted antenna supplies high frequency power. Sato has a disk-shaped conductor that performs impedance matching. A magnetic circuit using permanent magnets is provided close to the electromagnetic wave emitter.
DeOrnellas U.S. Pat. No. 6,190,496 claims a plasma etch reactor with a reactor chamber, two electrodes and power sources connected to the electrodes generate power at two different frequencies. A third electrode generates power at a low frequency. Magnetic confinement is utilized with the reactor chamber. The reactor is useful in etching the new class of films used in chip designs. The magnets can be permanent or electromagnets and are designed to concentrate the plasma which can reduce erosion to the electrodes and also protect the process chamber parts.
Yokogawa U.S. Pat. No. 6,033,481 discusses generation of uniform plasma over a large range for use in etching processing. High density plasma is generated in a vacuum vessel housing an electron cyclotron resonance device. An electromagnetic wave is radiated from a planar conductive plate arranged opposite to the surface of the sample being processed inside the vacuum vessel. The same patent also claims an electromagnetic wave radiation antenna consisting of strip-lines provided on an earth electrode opposite the processed sample.
Yokogawa U.S. Pat. No. 5,891,252 is similar to U.S. Pat. No. 6,033,481 and is cited because it too discusses antennas with a plasma source. The earlier patent provides a plasma processing apparatus which does not require large power consumption. It also discusses supplying an electromagnetic wave from a power source to a conductive plate in a planar shape and radiating the electromagnetic wave for forming plasma from the conductive plate.
The above-cited references do not disclose the particular combination of innovative features of present invention described hereinbelow. In Sato, et al., the design of the plasma generation antenna is the main part of the invention.